Sound reproducing system



`Octg 2l, 1941. B. oLNEY 2,259,907

SOUND REPRODUCINGr` SYSTEM Filed Nov. "s, 1195s NNN I N @E J EF BY Patented Oct. 21, 1941 UNITED STATES vPATEN T OFFICE SOUND REPRODUCING SYSTEM Benjamin Olney, Brighton, N. Y., assignor to The Stromberg-Carlson Telephone Manufacturing Company, Rochester, N, Y., a corporation of New York Application November a, 1939, serial No. 302,729

4 claims. (ci. 181-31) This invention relates to sound reproducing systems and more particularly to sound reproducing systems of the dual loud speaker type in which one loud speaker isv adapted to reproduce audio signals in the lower portion of the audio f frequency range and the second loud speaker is adapted toreproduce audio signalsin theremaining portion of the audio frequency range.

In some types of such sound reproducing systems, it has been thepractice in the past to employ a relatively large baille having one opening therein through which ylower frequency sound Waves generated by the front or hollow surface of therconical diaphragm of a low frequency response dynamic loud speaker are pror jected, and having a second opening spaced from thefirst thru which-the higher frequency sound waves generated by the front or hollow surface of a smaller conical diaphragm of a high frequency response dynamic loud speaker are projected. While such systems have a flatter audio frequency response characteristic with higher power handling capacity than is possible by the yuse of a single loud speaker designed to repro- `cluce all frequencies in the audio frequency range, nevertheless such dual loud speaker reproducing systems have failed to producethe expected fidelity and naturalness of sound reproduction, particularly in the case of speech and especially when the input energy is divided between the loud speakers at a frequency near the-middle of the speech range. As it is possible by careful design'to preserve piston action and flatness of response in low frequency cone loud speakers of moderate size up to about 1500 cycles per second, and as it is difficult if not impracticable to construct high frequency cone loud speakers which are suitable for frequencies below about 1500 cycles, at the same time preserving uniformity of response and desirable radiation.;

duced sound components reaching the ear, but

also produces the confusing impression of a double source.

While it is well known that substantial shifts -among the phase relations of the componentsr of a steady complex tone of moderate intensity 'are undetectable by the ear, the same is not true of transient sounds, the accurate reproduction of which is essential to the natural rendition of speech, music and noise. Although it `a multiple or an extended one. -when the speech energy is split into two frequency bands, each band being separately reprof duced by loud speakers in spaced side by side 45- invention.

is also well known thaty the phase relations of sounds at the ears of a listener maybe greatly modified byref'lections from walls and surrounding objects, it nevertheless is true ythat the ear can discriminate to a marked degree between the direct sound from a source and that reflected from the walls of a room.. `This is particularly true at the higher frequencies where the ears,

as well as most loud speakers, possess considerable directivity, and is also true for transient sounds. Asa matter ofy fact, the ear gathers its impression of the latter type of sound almost entirely `from the components yarriving directly from the source. It has furthermore been 'proved experimentally that even when two ordinary loudspeakers of identical characteristics 'are mounted side by side and as closely adjacent as their construction permits, the ear has no difficulty in determining from which loud speak- :er the sound comes when listening under usual home conditions `of distance from the sources land room reverberation.

From the above considerations, it may be. seen that the side by side spaced relation of the ttwo loudspeakers of adual system may produce the following undesirable eifects. First, because `of possible differences in the path length from the two loud speakers to the ear of the listener, phase .shift may occur'among the frequency components of a reproduced sound, resulting in audible distortion in the case of transients.' Impartance is lent to this by the fact that transients occur naturally at either or both the start and the finish of sounds produced by many musical instruments, that speech is very rich in transients, and that many types .of impulsive sou-nds are entirely transient in character.

Second, rone intuitively expects speech sounds .to issue from a single, relatively small source and'is disturbed when the source appears to be For example,

relation, a confusing and unnatural effect is experienced by the listener. He may hear vone A group of lower frequency components as issuing from one Ysource and a group of higher frequency components as issuing simultaneously 50` from another source appreciably distant from the first. 'These subjective effects are of a subtle nature and are difficult to analyze but, nevertheless, are believed tok account for the readily I observable difference between dual loud speaker 55y systems of the kind discussed above and the improved system which is the subjectv of this The arrangement of the two loud speakers Lin rspacial side by side relation has previously been an "unavoidable requirement in such systems diaphragm with a resultant serious reductionA in the response at the lower end of the desiredV frequency range.

It has long been recognized "that thelhigh frequency loud speakers of all such reproducing systems are subjected to large external acoustic driving forces which arise'from the sound pressures set up by the low frequency loud speakers. These pressures become especially high .in systems wherein the rear of the low frequency speaker is separately enclosed, and may be of sufcient magnitude to damage the high frequency loud Lspeaker unless protective measures are taken against such a contingency. lIn current systems, thisrprotection has taken the form of increased stiffness of the center or apex suspension of the diaphragm of the high frequency loud speaker. However, sufiicient stiffness for adequate protection not only results in a value which is detrimental to normal operation of the high frequency speaker,.but has the added disadvantage of concentrating at the center of the cone the reaction forces due to external acoustic driving, thus exposing the cone itself. to forces which may result in its deformation.

In accordance withthe main feature of the present invention, there is provided a sound reproducing system in which the loud speaker or reproducer designed to reproduce sound in the higher portion of the audio frequency range is mounted in coaxial superimposed relation with the loud speaker or reproducer designed to reproduce sound in the lowerportion ofthe audio frequency range whereby the effect of a unit sound source is obtained.

Another feature of the dual reproducing system of this invention resides in the use therein of a small diameter conical diaphragm high frequency loud speaker, the cone diameter being of the order of two and one half inches. A small diaphragm is desirable here-for several reasons, among which are: (l) the extensionof thehigh frequency range due to the smaller mass of the small diaphragm; (2) the preservation of piston motion to a higher frequency, thus contributing to atness of response, and (3) the consequent elevation of the frequencyrange in which wave motion in the cone structurey takes place to a region wherein the damping contributed by the cone material itself and its leather supporting edge is high and effective in suppressing the resonances concomitant with such Wave motion; also, (4) sharply directional radiation is postponed to such a high frequency that the deflecting vanes, used in former systems of this type to distribute the radiation, may be dispensed with.

Such small diaphragms have not'been successfully employed before as direct radiators in high frequency loud speakers of dual systems in which advantageis taken of themaximum ob-.V

tainable at response of the low frequency loud -speaker as previously described, because the acoustic radiation resistance of such small diaphragms falls oif vrapidly at frequencies considt erably higher than the optimum crossover fre;

quency.` For example, the radiation resistance of a two and one-half inch diameter .diaphragm in a baffle begins to fall off rat about 3400 cycles per second, whereas flat response may be desired down to'l500 cycles, 'I'his results in suchJa re:

v teristic impedance.

Nspeaker diaphragm so that the stiffness of the entrapped air therein together with the stiffness of the cone diaphragm supports of this loud speaker resonates broadly the mass of the cone diaphragm system at a frequency in the neighborhood of the lower portion of its frequency range. By thus increasing the velocity per unit force of the diaphragm in the region wherein its 'acoustic efficiency falls offr due to the decrease in radiation resistance, the response is restored to a, level comparable with that obtainable from the system over the remainder of its frequency range.

Other features of the high frequency loud speaker arethe provision of a diaphragm center support which produces relatively loW damping and Va diaphragm edge support which produces relatively high damping. In some former types of cone loud speakers intended solely for the re,-

production of high audible frequencies, the center suspension of the cone has consisted of a spider or` disc of relatively thick brous material -having substantial internal damping by virtue of friction among its constituent fibres. The outer cone suspension inl such former loud speakers has often consisted of an annular flange-like extension of the cone material itself but of substantially reduced thickness, and which afforded little damping for the edge of the cone. Such construction was not conducive to efficient and accurate sound reproduction as will be pointed out. All of the vibratory energy applied to the cone by the driving coil must necessarily `flex the center support because of the rigid attachment of the periphery of the latter to the vtruncated apex of the cone at or near its junction to the driving coil. Therefore, any energy dissipated in the center suspension is unavailable for the production of sound. On the other hand, edge damping of the cone is desirable for the suppression or control of energy reaching the edge by virtue of flexural vibration propagated through the cone structure itself as wave motion. If little or no damping is provided at the outer margin of the cone, reflection of this Wave energy takes place and the motion is propagated inwardly toward the apex, interfering with the outwardly'moving waves and producing at manifest themselves in sharp variations of sound output with frequency. The provision of suitable edge damping results in the suppression of such resonances and the more efficient transmission of the vibratory energy through the cone; a` result similar to that obtained by the termination of an electrical transmission line in its charac- In the present invention, the center suspension of the cone consists of a flexible disc of thin metal having 10W internal damping, such as one of the harder grades ofV duralumin, while the outer edge of the cone is suspended by means of an annulus of soft leather, suchas carpincho, having high internal damping. ,This combination results in an eflicientv diaphragm system pro- -ducinga very fiat response-frequency characteristic throughout its intended range.

A further feature of the invention relates to quency loud speaker. Furthermore, the enclosure of the rear surface of the high frequency loud speaker prevents circulation` between the front and back of the cone diaphragm thereof and thus the response at the lower end of-its desired frequency range is not reduced.

Other features and various advantages of the invention willv appear from the detailed description and claims when taken with the drawing in which Fig. 1 is a front view of the dual sound reproducing system kof the present invention with a portion thereof broken away;

Fig. 2 is a vertical section thereof taken substantially on the line 2,-2 of- Fig. 1 and Fig. 3 is a diagrammatic showing of a dividing network or audio frequency input circuit suitablel for actuating the dual loud speakers.

Referring especially tc Figs. 1 and 2, 5 designates a bale having an opening 6 therethru. This opening inthe present instance is of a size to function with an. eight inch diameter conev loud speaker, although it will be understood that the invention is not limited to this dimension. Behind this opening in the baiiie, there is mounted a conventional moving coil type of loud speak- I er l provided with a well-known cone shaped diaphragm 8 of paper or like material. This diaphragm, asis well known, is in the 'shape of a truncated cone having a moving coil 9 secured to its truncated portion and mounted to oscillate in a radial field produced by a well known magnetic structurelll.l This magnetic structure includes a cylindrical core. to a boss on the free alsol includes a magneticstruoture 22 for establishing a radial magnetic field through a moving coil- 23 attached tothe truncated apex of a coneshaped diaphragm 24 made of paper or like material. It will be understood that the truncated apex of the cone is supported by a flexible metallic disc suspension 23' of low internal damping fastened to a boss on the end lof the cylindrical core (not shown) of the magneticstructure 22. The harder grades of duralumin are suitable for the disc suspension. lThe outer edge of this cone diaphragm is supported by a iiexible suspension or annulus 25 of carpincho leather or the like, having relatively high damping, this suspension in turn being secured between clamping rings 26 and 2l. These clamping rings are secured to the periphery of acone-shapedI metal housing '28 completely enclosing the back surface of the ydiaphragm and being attached to the frontv of the magnetic structure .22. It will be understood that this housing is spaced from the diaphragm a distance suflicient to enable the diaphragm 24 to have its maximum movement without striking the housing. This housing together with the rear'surface of the diaphragm provides a chamber 29 in which airis trapped, the stiffness of which functions asa cushion to prevent the sound pressures developed by the low frequency loud speaker from moving the diaphragm system end of which the truncated portion of the cone diaphragm and the moving coil are resiliently mounted by a flexible center suspension generally designated vI2. The outer margin of the cone-shaped diaphragm is supported by the usual flexible suspension |13 of carpincho leather or the like which lsuspension is fastened between the clamping rings yHl and l5 and serves with the baflie to prevent circulation between the front and rear of the diaphragm. kThese clamping vrings, which are fastened by bolts I6 to the rear surface of the baille are supported on a bracket l|"|, 'secured tothe front ofthe magnetic structureito function as a support therefor. The rear Asurface of the diaphragm S communicates with a `speaker so that the bass response of this speaker Yis augmented thereby.

A high frequency cone-type loud speaker 2| of small size kbut of essentially the same structure as that of the low frequency loud speaker, is mounted coaxially with the diaphragm of the first loud speaker so that it nests in the hollow of said diaphragm. This high frequency loud speaker sufficiently to damage it. As this stiffness is distributed uniformly over the diaphragm surface, there is no tendency toward deformation of this vdiaphragm such as might result Iwere sunicient `mecharn'cal stiffness for protective purposes concentrated, for example, in the center suspension of the diaphragm. The stiffness of the system, of course, becomes ineffective above resonance and offers no impediment to the operation of the speaker within its intended frequency range. This entrapped `air lalso provides the stiffnesselement which cooperates resonantly with the mass of the moving system, as previously described, to offset 4the falling oif of Aacoustic radiation 'resistance.

The high frequency loud speakerunit is supported by means of the arms 30 in nested coaxial relation with the cone diaphragm of the low frequency loud speaker. The ends of these arms areirespectively bolted to the magnetic structure 22 and to the rear surface of thev baffle `5, the main fastening to the baiiie being accomplished by the same bolts I6 that mount the low frequency loud speaker. The baiie is preferably recessed at its rear surface to receive the outer ends of the arms 30 thereby enabling these arms to lie flush sov that no obstruction is loffered to the sealing of the. low frequency cone rim to the baille. It will be noted that the outer ends of the arms 30 are secured to the rear surfacey of the baiiie independently of the low frequency loud .speaker by the screws 3| so that support for the high frequency loud speaker is not lost during installation or removal of the low frequency -1oud` speaker. Because both .loud speakers are supported from theback of the baffle, the usual grille coils of the loud speaker units .1 and 2| will be connected into an audio frequency dividing. network, as shown in Fig. 3, the terminals 33 and 34 being connected to a suitable source of audio frequency currents such as the output of a radio receiver. The magnetic structures of these' loud speakers are arranged to develop suitable radial magnetic fields through the driving coils of these devices either by including permanent magnets therein or by electromagnets energized from a suitable direct current source in accordance with the well-known manner.

It might be expected that the high frequency loud speaker, since it is mounted directly in front of the cone diaphragm of the low frequency loud speaker, would constitute an undesirable obstruction to the radiation of the cone diaphragm of the low frequency loud speaker. In the present invention any existing effect of this nature is turned to an advantage by properly proportioning the size of the two loud speakers with respect to each other and to the point in the audio frequency range where the low frequency loud speaker ceases to radiate sound and the high frequency loud speaker starts its response. This crossover frequency in the present system is in the neighborhood of 1500 cycles. It may be shown from the work of Stenzel (Elektrische Nachrichten-Technik, June, 1927) that a cone diaphragm functioning as a piston radiator becomes effectively directive when its diameter approaches a half wave length. In the preferred form of the present invention the low frequency cone diaphragm is eight inches in diameter so that it increasingly tends to produce beam radiation from about 850 cycles upward. Furthermore, it may be deduced from the analysis given by Lord Rayleigh (Theory of Sound, vol. 2, page 272) and others,v that a spherical obstacle in the path of a sound wave becomes effectively dif- 'fractive as its diameter approaches one-quarter of a wave length. The present high frequency loud speaker structure with its clamping rings and driving unit effectively constitutes an obstacle roughly equivalent to a sphere four inches in diameter, and therefore becomes an effective diffractor at about 850 cycles.` This is the frequency at which the low frequency speaker begins to manifest directional effects. Above 850 cycles and up to the cutoff point of the low frequency speaker, the high frequency speaker structure acts as a deflector or spreader for the radiation of the low frequency loud speaker and thereby functions usefully in causing the latter to maintain a more uniform sound field. In order to satisfy the above conditions it will be noted that the diameter of the effective sphere correspending to the high frequency loud speaker as an obstacle should be one half that of the diaphragm of the low frequency loud speaker.

The onset of directional radiation in the present system at frequencies within the range of the high frequency loud speaker is postponed to a frequency in the neighborhood of 3000 cycles by making the diaphragm of the high frequency loud speaker of small diameter, only two and one-half inches in the preferred embodiment of this invention. Thus the frequency range over which wide angle sound radiation takes place, is much greater than can be had with a single loud speaker of sufiiciently great diameter to radiate low frequencies effectively. This feature constitutes another advantage of the present dual system.

What I claim is:

1. In a sound reproducing system, a loud speaker of the cone diaphragm type designed to reproduce Asound in the lower portion of the audio frequency range, a second loud speaker of the cone diaphragm type designed to reproduce sound in the higher portion of the audio frequency range, said last-mentioned diaphragm being exibly supported at its edge and at its apex, said second loud speaker being nested in the hollow of the diaphragm of said first loud speaker and means for resonating said second loud speaker in the lower part of the audio frequency range reproduced by it, said resonating means comprising the mass ofthe exibly supported portion of said second loud speaker together with the stiffness of the confined air adjacent the back surface of the diaphragm of said second loud speaker.

2. In a dual sound reproducing system comprising a low frequency loud speaker of the cone diaphragm type and a high frequency loud speaker of the cone diaphragm type, said loud speakers being mounted in nested coaxial relation, the cone diaphragm of said high frequency loud speaker having a centersuspension and a peripheral annular suspension, said center suspension having negligible damping and said peripheral suspension having relatively high damping, and means for substantially enclosing the back surface of the cone diaphragm of said high frequency loud speaker to afford therewith a chamber having a cushion of entrapped air therein.

3. In a sound reproducing system, a loud speaker of the cone diaphragm type designed to reproduce sound in the lower portion of the audio frequency range up to a cross-over frequency, a. second loud speaker including a cone diaphragm designed to reproduce sound in the higher portion of the audio frequency range down to said crossover frequency, the diameter'of said last-mentioned diaphragm being such that the radiation resistance thereof is seriously deficient in the lower portion of its audio frequency range, said second loud speaker being nested in the hollow of the diaphragm of said first loudspeaker and resonating means for augmenting the response in the lower part of each of the audio frequency ranges reproduced by said loud speakers, said means including means for increasing the velocity per unit force of said second diaphragm in the range where its radiation is seriously deficient, each loud speaker being provided with a field magnet separate from the field magnet of vthe other loud speaker.

4. In a dual sound reproducing system comprising a low frequency loud speaker of the cone diaphragm type designed to reproduce sound in the lower portion of the audio frequency range up to a cross-over frequency, and a high frequency loud speaker of the cone diaphragm type designed to reproduce sound in the higher por- -tion of the audio frequency range down to said cross-over frequency, said high frequency loud speaker being mounted in nested coaxial relation with said low frequency speaker, said high vfrequency loud speaker having an overall diameter approximately equal to one-half of the diameter of the low frequency cone diaphragm, whereby said high frequency speaker will noticeably spread the radiation of the low frequency speaker in the region of said cross-over frequency.

BENJAMIN OLNEY. 

